This invention relates to purification of aqueous solution containing soluble inorganic materials and more particularly, this invention relates to more efficient or enhanced membrane filtration of such solutions.
One of the problems with membrane filtration such as reverse osmosis filtration of aqueous solutions containing soluble inorganic material is the clogging of the membrane. That is, one of the most serious technical limitations of using reverse osmosis membranes to achieve high water recovery from a natural water source such as water wells or aquafers, municipal water supply or industrial wastewater stream is the potential precipitation of insoluble or sparingly soluble inorganic salts that form scale on the membrane as these compounds are concentrated many-folds (typically 10-20 times, not withstanding osmotic pressure limitations). These scalants (scale forming materials) can be the carbonate, sulphate or fluoride salts of calcium, barium, strontium and magnesium. Soluble silica is also a known scalant forming material because solubility is on the order of 100 mg/L at standard conditions of temperature and pH. In addition, RO membrane fouling due to ferric hydroxide formation at the surface of the membrane has been a problem for many applications because iron is present in most industrial waters.
For purposes of purifying aqueous streams contaminated with organic materials, U.S. Pat. No. 4,000,065 discloses the use of a combination of reverse osmosis (RO) and ultrafiltration (UF) to separate the organic material from the aqueous stream. The contaminated aqueous stream is circulated from the high pressure compartment of a RO unit to the high pressure compartment of an ultrafiltration unit, then to the low pressure compartment of the ultrafiltration unit, and then back to the high pressure compartment of the RO unit.
Japanese Patent 57-197085 discloses a filtration apparatus that comprises connecting ultrafiltration apparatus and a reverse osmosis apparatus in series so as not to deposit scale on the osmosis membrane.
U.S. Pat. No. 3,799,806 discloses purification of sugar juices by repeated ultrafiltration and reverse osmosis.
U.S. Pat. No. 4,083,779 discloses a process for treatment of anthocyante extracts by ultrafiltration and reverse osmosis treatments.
U.S. Pat. No. 4,775,477 discloses a process for extraction of cranberry presscake wherein the presscake is ground and subjected to microfiltration to remove colloidal high molecular weight compounds followed by reverse osmosis to recover a red-colored solution.
U.S. Pat. No. 5,182,023 discloses a process for removing arsenic from water wherein the water is first filtered to remove solids then passed through an ultrafilter, followed by a chemical treatment to adjust pH to a range from about 6 to 8. Thereafter, anti-scalants and anti-fouling materials are added before subjecting the water to reverse osmosis to provide a stream having less than about 50 PPB arsenic.
Japanese Patent 53025-280 discloses the separation of inorganic and organic compounds from a liquid by first using a reverse osmosis membrane and then using a second reverse osmosis membrane having a more permeable membrane such as a microporous or ultrafiltration membrane. Part of the concentrated liquid obtained from the first membrane is processed through the second membrane.
To minimize the effect of insoluble or sparingly soluble salts that result in scalants or foulants that affect the reverse osmosis membrane, many different treatments have been used. Treatments include filtration, aeration to remove iron, and pH adjustment to prevent carbonate scale formation. Anti-scalants such as polyacrylic acid, EDTA and sodium hexametaphosphates are used to control formation of divalent metal fluorides, sulphates, silica and residual iron hydroxides. However, this approach is often not practical because of the cost of the anti-scalants. In addition, many times the anti-scalants are not compatible in certain applications. Further, they often are required to operate in a narrow pH range and are not tolerant of other materials, e.g., polymeric anti-scalants usually are not effective in the presence of iron. Also, in many cases, the antiscalants are not effective when the concentration of sparingly soluble salts exceeds the saturation solubility by many fold (e.g., 5-10 times). Thus, it will be seen that this approach alone is not very desirable because it has only limited effectiveness.
For purposes of the present invention, foulants are regarded as solids contained in the liquid that accumulate on the surface of the reverse osmosis membrane. Scalants are regarded as sparingly soluble inorganic compounds that can exceed their solubility limit and can result in a concentration gradient across the thickness of the reverse osmosis membrane. Scalants or sparingly soluble inorganic compounds can precipitate or deposit in situ of the membrane in small crystals that bind tightly to membrane surfaces making their removal extremely difficult, if not impossible, and severely limit flow of liquid through the membrane. Often, solution or chemicals required to remove or dissolve the scalants adversely affect the membrane. Thus, the formation of scale can lead to the eventual loss of the membrane.
Thus, there is a great need for a process or system that will enhance the useful life of the reverse osmosis membrane by removing insoluble or sparingly soluble inorganic salts from an aqueous solution and yet permit 90% to 99% liquid recovery in an economical manner.